1x8 instead of plywood deck

I have an old house. The decking of the roof is 1x8 boards (maybe they are 1x6 not 100%), however, it is not larger pieces of plywood like you would see on a modern home. The tar paper is applied over these boards and then shingled over. The roof was last shingled about 7 years ago. Im trying to figure out if that is wrong. Should the boards have been covered with plywood?